1. Field of the Invention
The present invention relates to the field of direct bonded metallic/substrate structures, and more particularly, to thermally matched structures of this type.
2. Background Information
In semiconductor device packaging and other arts, there is often a need to form a bond between a metallic and a ceramic structure or between two metallic structures which provides a hermetic seal and which will withstand high processing and operating temperatures. The direct bond copper process which is described in U.S. Pat. Nos. 3,744,120, 3,854,892 and 3,911,553 to Burgess et al.; U.S. Pat. Nos. 3,766,634 and 3,993,411 to Babcock et al.; U.S. Pat. Nos. 3,994,430 and 4,129,243 to Cusano et al.; U.S. Pat. No. 4,409,278 to Jochym; and U.S. Pat. No. 4,563,383 to Kuneman et al. met this need by forming a copper oxygen eutectic mixture which wets both metallic copper and ceramic materials such as alumina and beryllia and which bonded the members together upon solidification. This process has been in use for many years. Each of these patents is incorporated herein by reference.
The basic ones of these patents (3,766,634; 3,993,411; 3,744,120 and 3,854,892) each state that "One factor which appears to affect the tenacity and uniformity of the bond is the relationship between the melting point of the metallic member and the eutectic temperature. Where the eutectic temperature is within approximately 30.degree. to 50.degree. C., of the melting point of the metallic member, for example, the metallic member tends to plastically conform to the shape of the substrate member and thereby produce better bonds than those eutectics which become liquids at temperatures greater than approximately 50.degree. C. below the melting point of the metallic member. The uniformity of the bond, therefore, appears to be related to the `creep` of the metal which becomes considerable only near the melting point. From Table I [not reproduced here], for example, it can be seen that the following eutectic compounds meet this requirement: copper-copper oxide, nickel-nickel oxide, cobalt-cobalt oxide, iron-iron oxide and copper-copper sulfide." Thus, it is considered important in this field that the metallic member be near its melting point during the bonding process in order that the metallic member may be conformed to the surface of the substrate by the tension of the liquid eutectic mixture.
As a need for larger and larger substrates and packages has developed, the use of direct bonded copper in combination with ceramic substrates has created a thermal mismatch problem in that copper has a relative thermal coefficient of expansion of about 16, while alumina has a relative thermal coefficient of expansion of about 7, and beryllia has a relative thermal coefficient of expansion of about 8. Since the eutectic bonds in a copper/copper oxide eutectic structure are formed at 1,065.degree. C., substantial stress and warping is induced in large structures during the cooling process. A number of techniques have been employed in an attempt to overcome this problem. In one of these techniques, rather than a continuous copper layer, a matrix of smaller copper units has been bonded to the ceramic to avoid a large expanse of copper bonded to the ceramic which tends to form a "bimetallic" strip. As another alternative, a large area copper foil was formed with hoops between flat sections with only the flat sections being direct bonded to the ceramic with the hoops providing strain relief. Both of these techniques have the drawbacks that they result in a more complicated structure than a single continuously bonded copper layer would provide (if not for the thermal mismatch problem), a more complex fabrication procedure is involved and in the latter case, a non-planar surface results. There is a need for a direct bond metallic ceramic structure which is substantially free of thermal mismatch.
U.S. Pat. 4,563,383 to Kuneman et al. which is cited above, discloses a method of fabricating a ceramic/metallic laminate structure which is free of thermal coefficient of expansion mismatch-induced bending. Two substantially identical ceramic substrates each have a thin copper foil eutectic bonded to one surface thereof. Thereafter, these two substrates are placed on opposite sides of a central copper foil with their copper-coated sides toward the foil and a eutectic bond is formed between the central foil and each of the foils already bonded to the ceramic. Alternatively, all three eutectic bonds may be formed in a single heating step. The resulting structure is symmetric and, therefore, free of thermal coefficient of expansion mismatch induced bending. While the resulting ceramic substrate is beneficial for appropriate applications, it does not solve the problem of thermal coefficient of expansion mismatch-induced bending in ceramic/metallic structures which by their nature, must be or are asymmetric.
A symmetric bimetallic laminate of copper-molybdenum-copper (Cu-Mo-Cu) has been developed to provide metallic members having thermal coefficients of expansion which are between the thermal coefficients of expansion of copper and molybdenum and which are free of thermal coefficient of expansion mismatch-induced bending. The symmetric bimetallic foils differ from typical bimetallic strips which are intended to bend in response to changes in temperature, in that a central core of molybdenum is provided with two identical layers of copper each laminated to a different side of the core so that a symmetric sandwich structure results in which the copper is the bread of the sandwich and the molybdenum is the filling of the sandwich. Since the bread layers have the same composition and the same thickness, the stresses induced by thermal coefficient of expansion mismatch are equal on both sides of the core layer and no bending is induced. This symmetric bimetallic laminate finds application in those situations where a thermal coefficient of expansion intermediate that of the bread and sandwich layers is desired. Other such laminates such as copper-tungsten-copper would also be useful. The Cu-Mo-Cu laminate is formed by rolling and is readily available. We have been unsuccessful in finding roller laminated Cu-W-Cu. Cu-W-Cu has been reported to be available in which the tungsten is a matrix or grid and the copper layers cover both side of the tungsten and merge in the holes in the matrix or grid in what is known as an infiltrated copper structure.
There is a need for a method of fabricating metallic/ceramic composites which are asymmetric and substantially free of thermal coefficient of expansion mismatch-induced bending.